1. Field of the Invention
The present invention relates to an electromagnetic field intensity calculating method and a storage medium for storing a program for executing the method. The electromagnetic wave is emitted from a device such as a printed circuit board, a body of a vehicle, or a rear window of a vehicle in which an antenna is embedded.
The strength of the electromagnetic wave emitted from electronic devices must be suppressed within a certain permitted range, regardless of whether the electromagnetic emission is necessary or not. To this end, it is necessary to calculate, by simulation, the intensity of the electromagnetic field considered to be emitted from a device under consideration.
2. Description of the Related Art
In a conventional electromagnetic field calculating method, the surface of the device under consideration is virtually divided into a mesh of a plurality of small elements. Hereinafter, the small elements are referred to as shell elements. Each of the shell elements has a rectangular shape, and the intensity of the electromagnetic field emission is calculated by estimating a current considered to flow through each shell element.
There is, however, a problem in that the intensity of the electromagnetic emission cannot be calculated when the shell element has the shape of a triangle such as one at the corner of the device under consideration. This problem will be described with reference to FIG. 9 and FIG. 10.
FIG. 9 is a diagram showing a part of the surface of a device under consideration in which the surface is virtually divided into a mesh.
In the figure, all of the shell elements, other than the shell elements 91 and 92 each having a shape of a triangle, are rectangular. An electric current, which is considered to flow each side of a rectangular shape of a shell element, can be considered to continuously flow across the opposite side of the shell element. Therefore, the current can be calculated according to the well-known moment method. The vector quantities of the currents are illustrated in the figure by solid arrows.
On the other hand, the quantity of a current considered to flow from each side of a shell element 91 or 92 having a triangular shape toward the apex of the triangle can be calculated only when there is a current route from the apex to a point other than the apex. When there is no current route from the apex to a point other than the apex, the current quantity must be set to zero. For example, when a wire element 95 is connected to the apex 95 of the triangular shell element 92, the current considered to flow across the side 94 can be calculated. However, when nothing is connected to the apex such as the apex 96 of the triangular shell element 91, the current considered to flow across the side 94 must be set to zero.
FIG. 10 is a diagram showing an electric current flowing through a certain pattern. The current is calculated in accordance with the conventional method of calculating electromagnetic field intensity. The device under consideration as illustrated in the figure has a pattern width of 1 mm, a pattern interval of 2 mm, and a characteristic impedance of 294xcexa9. When a current flowing through this device is calculated in accordance with the conventional electromagnetic field intensity calculating method under the condition that a current of 200 mA with a frequency of 30 MHz is supplied from a power supply 85 to this device, the calculated current will be zero. This is because, in triangular shell elements 101, 102, 103, and 104 which are present in the corners of the pattern, it is assumed that the current across each side to its opposite apex is zero. In practice, however, a certain finite current must flow through the device. Therefore, in case of the device as shown in FIG. 10, the actual intensity of the electromagnetic emission can not be calculated in accordance with the conventional electromagnetic field intensity calculating method.
An object of the present invention is to provide an electromagnetic field intensity calculating method which can calculate the electromagnetic field intensity emitted from a device under consideration even when the shape of a shell element is triangle, and even when a wire element is not connected to an apex of the triangle.
To attain the above object, there is provided, according to the present invention, an electromagnetic field intensity calculating method for calculating the intensity of electromagnetic field emitted from a device under consideration, by using a moment method in which a mutual impedance between shell elements and a supplied voltage are assumed to be known quantities and the current flowing through the shell element is assumed to be an unknown quantity. The method comprises the steps of obtaining a plurality of shell elements including at least one triangular shell element by virtually dividing the device under consideration into mesh, and setting an unknown quantity of a current directing from each side of the triangular shell element toward its apex only when a conductive wire is connected to said apex, setting an unknown quantity of a current directing from each side of the triangular element toward its apex when said side is common with a side of the other shell element, or setting an unknown quantity of a current parallel to a side of the triangular shell element when there is a shell element adjacent to the triangular shell element.
For all of the three congruent triangles, the step of setting an unknown quantity of a current repeats the steps of selecting one shell element from the plurality of shell elements, determining the shape of the selected shell element, preparing, when the selected shell element is judged to be triangle as a result of the judgement, three triangles which are congruent with the selected triangular shell element, making a correspondence between one of apexes of each of the triangles and a starting point of a current which is assumed to direct to the outside of each of the three congruent triangles in such a way that the starting point of one of the congruent triangles corresponds to an apex of the one of the congruent triangles, and, when the corresponding apexes of the three congruent triangles overlap, the position of the starting point of one of the triangle is different from any one of the positions of the other starting points of the other triangle, selecting, in a triangle, a side opposite to one of the starting points, judging whether or not a conductive element is connected to the starting point opposite to the selected side, setting, when it is judged that a conductive element is connected to the starting point opposite to the selected side, an unknown quantity of a current directing from the selected side toward the starting point, judging, when it is judged that the conductive element is not connected to the starting point of the selected triangle, whether or not there is a shell element adjacent to a side other than the selected side of the selected triangle shell element, setting, when it is judged that there is not a shell element adjacent to a side other than the selected side of the selected triangle shell element, an unknown quantity of a current directing from the selected side toward the starting point, and setting, when it is judged that there is a shell element adjacent to a side other than the selected side of the selected shell element, an unknown quantity of a current parallel to one side of the triangle in such a way that a current flows into the adjacent shell element.
The unknown quantity of the current parallel to one side of the triangle is obtained by integrating the unknown quantity of the current parallel to the side from each side to the apex opposite to the side except for the apex.